<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <div id="app">
    <input type="text" v-model='msg'>
    {{ msg }}
  </div>
</body>
<script src="./vue.js"></script>
<script>
  const vm = new Vue({
    el: '#app',
    data: {
      msg: 'hello vue'
    },
    watch: {
      msg(newVal,oldVal){
        console.log('msg值变化了',newVal,oldVal);
      }
    }
  })
</script>
</html>